1. Technical Field
This invention relates to portable telephones and more particularly to a portable telephone arranged for communicating in a frequency hopping system
2. Description of the Prior Art
The Federal Communications Commission (FCC) recently promulgated rulings in the utilization of spread spectrum systems, including a frequency hopping system. These rulings now allow for increased spectral utilization by portable telephones. Frequency hopping systems spread their energy by changing, or hopping the center frequency of the transmission many times a second in accordance with a pseudo-randomly generated list of communication channels. The result is a significantly higher signal to noise ratio than may be achieved by conventional techniques such as amplitude modulation that uses no bandwidth spreading.
Digital portable telephones are now being designed for use in spread spectrum systems, including frequency hopping systems. Examples of such digital portable telephones are described in U.S. Pat. No. 5,323,447 which issued to M. E. Gillis et al. on Jun. 21, 1994, U.S. Pat. No. 5,353,341 which issued to M. E. Gillis et al. on Oct. 4, 1994 and U.S. Pat. No. 5,463,659 which issued to W. J. Nealon et al. on Oct. 31, 1995. Each of these digital portable telephones utilizes time division duplexing for communicating between a base unit and its associated handset unit. In the operation of such a portable telephone, the base unit and the handset unit communicate with each other by alternately transmitting and receiving a signal on the same channel. Thus, while the base unit is transmitting on a channel, the handset unit is configured for receiving the signal from the base unit over that same channel. Similarly, while the handset unit is transmitting on the channel, the base unit is configured for receiving the signal from the handset unit. Due to the packetizing handling of voice data in these digital portable telephones, delays which result in echoes discernible by the user of the handset unit may be generated at certain packetizing rates of the voice data.
The effect of such echoes is minimized by employing an echo suppression arrangement such as is described in U.S. Pat. No. 5,381,475 which issued to M. C. Cavallo on Jan. 10, 1995. This suppression arrangement attenuates the reflected voice signal from the handset unit appearing in the base unit, and thereby prevents the sidetone echo from being coupled to and emanating at the handset unit. Although such arrangement effectively suppresses the echoes, it is desirable to design a simpler digital portable telephone which does not employ time division duplexing.